South Atlantic Cenozoic Paleoceanography

Abstract

Several suites of undisturbed cores obtained by continuous hydraulic piston coring provided material for the investigation of Cenozoic paleoceanography. The faunal record is complete, except for the early Eocene. The isotope records have gaps in the early Eocene and in the early and middle Miocene. Hundreds of samples were studied and thousands of analyses were carried out. Detailed results and conclusions are presented in several separate chapters of this volume. This report is a synthesis.Studies of calcite dissolution in pelagic sediments indicate that calcite compensation depth (CCD) and lysocline underwent two types of changes during the Cenozoic. The first-order changes, which have a periodicity of some I0 7 yr., have amplitudes (depth difference from the medium) of more than 1,000 m. The Eocene and Miocene were epochs of high CCD. The deepening of the CCD in the late Eocene and early Oligocene was stepwise, but it was more abrupt in the early Pliocene. We attribute the higher CCD in the Eocene and Miocene to a relatively low production rate of calcareous plankton in the open oceans; during those epochs many of the nutrients had been consumed by phosphate deposition and by siliceous plankton. The second-order changes, which have a periodicity of I0 4 or I0 5 yr., represent CCD variations in response to changes from interglacial to glacial paleoenvironments during the Pliocene and Quaternary; the amplitude was a few hundred meters only. Evidence suggests that increased dissolution due to the more active bottom waters during the glacial stages in the Atlantic far overshadowed the effect of possible increases in fertility.The oxygen-isotope record shows an overall increase of δ 18 θ values since the middle Eocene, with a moderate reversal of the general trend during the late Oligocene and early Miocene. Whereas the early Paleogene trend of the Atlantic middle-latitude sites closely simulated that of the southern oceans, vertical and latitudinal gradients began to develop in the Oligocene and continued to steepen with time, so that the present thermal structure of the ocean waters above our sites is similar to that of the equatorial Pacific. The cause of the increased δ 18 θ values should have been related to increased ice volume on Earth and to temperature declines. However, the relative importance of the two different factors at any given time cannot be ascertained. We favor the hypothesis that the Antarctic Ice Cap started to form in the late Eocene and suggest that the early Oligocene ice volume was larger than it is today. The middle Miocene oxygen shift has also been registered at one of our sites, but its magnitude is smaller, and the corresponding environmental changes should have been less impressive, than those in the late Eocene and early Oligocene.The carbon-isotope record shows a parallel trend between the planktonic and benthic foraminifers since the beginning of the Oligocene and a divergent trend as well as a steeper gradient during the middle Eocene. Peak values of δ 13 C are found in ...

Late Cenozoic Carbonate Accumulation and the History of the Carbonate Compensation Depth in the Western Equatorial Indian Ocean

“…Since then, most reconstructions of the CCD have shown a clear shoaling trend from the Oligocene into the Miocene (e.g., van Andel, 1975;Heath et al, 1977;Hsu et al, 1984;Rea and Leinen, 1985), a trend similar to the one seen here. The study by Rea and Leinen (1985) of nearequatorial South Pacific sites is of particular interest, however, because of their observations of carbonate accumulation above the CCD.…”

“…The principal paleoceanographic objective of Leg 115 was to use this new technology to recover a depth transect of continuously cored sedimentary sequences from the tropical Indian Ocean that would allow reconstruction of the dynamic features of the regional carbonate system through the late Cenozoic. Deep-sea drilling has long since established that dissolution levels in the different ocean basins have fluctuated markedly during the Cenozoic (e.g., Berger and Winterer, 1974;van Andel, 1975;Heath et al, 1977;Hsu et al, 1984;Rea and Leinen, 1985). In general, these studies have shown that long-term changes in the patterns of carbonate distribution have been similar between the major ocean basins, suggesting a global forcing mechanism perhaps linked to changing sea level or to a changing supply of carbonate to the oceans (Kennett, 1982).…”

Sediment-Mass Accumulation Rate and Variability in the East China Sea Detected by GRACE

“…Second, around the Congo estuary, the Congo Canyon immediately off Congo's west coast serves as a conduit for turbidity currents to carry away sediments from the Congo River to the oceans [53]. In addition, the branch of the northward flowing Benguela Current along the west coast of Africa can reduce sediment depositions on the northern side of the Congo River estuary [54], which may cause the potential MAR signal to emerge north of Congo's estuary in Figure 10b. Again, the GRACE data quality is another deciding factor: it is likely that GRACE data uncertainties in some of the estuaries in Figure 10 are so overwhelming that they suppress potential MAR signals.…”

Eocene and Oligocene Sporomorphs and Dinoflagellate Cysts from Leg 113 Drill Sites|Weddell Sea|Antarctica

“…Dating of the build-up of large ice-sheets on the Antarctic continent, with their strong feedback effects on the climatic development of the total globe, is still controversial (Hsu et al, 1984;Kennett, 1980;Webb et al, 1984;Robin, 1988).…”